1. Field of the Invention
The invention relates to the process for the gasification of coal in suspension wherein the produce gas, called synthesis gas or syngas, is cooled by feeding back cleaned and cooled product gas into the product gas as it leaves the gasifier unit.
2. Prior Art
Processes for the gasification of coal in suspension have been known since the 1940's. In order to avoid the fouling of heat transfer surfaces of the waste heat boilers used in a conventional process for the gasification of coal, it is necessary to solidify the liquid slag droplets that are entrained in the gas leaving the gasifier, and to cool the liquid slag droplets to a temperature at which they are not sticky. This means that the entire gas stream leaving the gasifier must be cooled to a temperature that is about 100.degree. F. (38.degree. C.) below the slag softening temperature. For most coals the softening temperature of the ash is in the range of about 1900.degree. F. to 2400.degree. F. (1037.degree. C. to 1316.degree. C.). It is customary to operate the gasifier at a temperature of about 2700.degree. F. (1482.degree. C.) and to quench the hot gas just as it leaves the gasifier but before it enters the waste heat boiler.
As shown in the U.S. Pat. No. 3,963,457 to Hess, the Koppers-Totzek process (KTP) is recognized and understood by those skilled in the art to be a process for the gasification of coal in suspension. Previous gasifiers, such as the KTP, utilized spray water from the primary water pump into the stream of product gas just as it left the gasifier in order to cool the product gas and solidify the liquid slag droplets entrained therein. The use of spray water caused a large heat loss in the product gas however and, to eliminate this large heat loss, Hess improved the process by recycling cleaned and cooled product gas back into the product gas as it leaves the gasifier unit thereby cooling the product gas and eliminating the need for water sprays. This improved the thermal efficiency by a significant amount.